A so-called subfield method is known as a method for displaying a multiple gradation image by using a display device for performing binary display of a plasma display panel, etc. The subfield method is a method in which one field of an image signal is constructed by plural subfields weighted by brightness, and gradation is displayed by performing coding for controlling light emission or light non-emission of a pixel of each subfield.
For example, one field of the image signal is divided into eight subfields, and the brightness weights of the respective subfields are set to “1”, “2”, “4”, “8”, “16”, “32”, “64” and “128”. The image signal is then set to a digital signal of eight bits, and this digital signal is sequentially allocated to the eight subfields from a least significant bit, and turning-on and turning-off control is performed so that images of 256 gradations can be displayed. However, when a dynamic image is displayed by the above display method, it is known that a great gradation disturbance of a contour shape, a so-called dynamic image false contour is generated in an area in which there is a movement within the image (hereinafter abbreviated as a “dynamic image area”).
Therefore, as one method for generating no dynamic image false contour, it is tried that the movement of the image is detected and the display method of a gradation value, i.e., a coding method is changed in accordance with the existence of the movement of the image. In this trial, for example, in an area having no movement of the image (hereinafter abbreviated as a “static image area”), the gradation values display the 256 gradations from “0” to “255” in the above method, and the display is performed in the dynamic image area by limiting the gradation values to gradation values difficult to generate the dynamic image false contour. The dynamic image false contour in the dynamic image area can be reduced by such a display method. Further, the gradations of 256 combinations can be displayed in the static image area.
In the case of gradation values at which the subfield turned on in a direction sequentially increased from a minimum subfield in brightness weight is continued, the gradation values difficult to generate the dynamic image false contour are nine gradation values of “0”, “1”, “3”, “7”, “15”, “31”, “63”, “127” and “255”.
FIG. 10 shows the nine gradation values difficult to generate the dynamic image false contour. Here, a “circular mark” shows the subfield turned on with respect to each gradation value. The generation of the dynamic image false contour can be restrained by limiting the gradation values to only these nine gradation values and displaying an image of the dynamic image area by using these gradation values. However, in this case, the number of gradations able to be displayed is only nine. Therefore, image display quality is extremely reduced in this case. Therefore, gradation is corrected by using a so-called error diffusing method in which the difference between the gradation value to be displayed and the gradation value actually displayed is diffused to circumferential pixels by an appropriate ratio.
FIG. 11 is an explanatory view of the error diffusing method in the prior art. In the pixels shown by hatching and a white circle of FIG. 11, an error generated between the gradation value to be displayed and the gradation value actually displayed is respectively divided into a pixel adjacent on the right-hand side, a rightward downward pixel, a pixel just below, and a leftward downward pixel in a ratio of 7:1:5:3, and is added. In each pixel, a value provided by adding the gradation value to be displayed and the diffused error is set to a gradation value to be newly displayed, and a gradation value closest to this gradation value to be newly displayed is selected from the above nine gradation values, and is set to the gradation value actually displayed. Thus, the error between the gradation value to be displayed and the gradation value actually displayed is diffused to the circumferential pixels. This processing is repeatedly performed so that the gradation values except for the above nine gradation values can be artificially displayed by using the nine gradation values.
FIG. 12 is a circuit block diagram of a conventional image display device. The conventional image display device has movement detecting section 102 for detecting the dynamic image area from an input image signal, and also has adding section 106 for adding an error diffused from a circumferential pixel to the input signal. The conventional image display device also has static image coding section 107 for performing coding with respect to a pixel of the static image area with respect to an image signal provided by adding the error, and also has dynamic image coding section 108 for performing coding with respect to a pixel of the dynamic image area with respect to the image signal provided by adding the error. The conventional image display device also has selector 109 for selecting one of outputs of static image coding section 107 and dynamic image coding section 108 in accordance with an output of movement detecting section 102, and also has subtracting section 110 for calculating the error between an input image and an output image. The conventional image display device also has multiplying section 111 for performing predetermined weighting with respect to the error, and also has delay section 112 for adjusting timing to diffuse the error to a predetermined pixel. The conventional image display device further has display section 113 for displaying the image signal. The conventional image display device executes the above error diffusing operation.
However, in such a conventional method, the coding method is switched at the boundary of the dynamic image area and the static image area. Therefore, there is a case in which a noise of a sharp edge shape (hereinafter called a “switching shock”) is generated at this boundary in accordance with an image. In particular, this switching shock is easily generated with respect to an image in which an object is moved with an area flat in brightness as a background.
In contrast to this, Japanese Patent Unexamined Publication No. 2003-69922 proposes a method for reducing the switching shock by diffusing a boundary portion by a random number and setting edges not to be uniformed. However, in the method described in this laid-open patent publication, the boundary of the dynamic image area and the static image area is merely diffused by using the random number. Accordingly, the boundary of the dynamic image area and the static image area is still left in the sharp edge shape, and no switching shock is perfectly vanished.